The Oil & Gas upstream production industry drills wells of ever increasing depth and complexity to find and produce raw hydrocarbons. The industry routinely uses steel pipe (Oil Country Tubular Goods) to protect the borehole (casing) and to control the fluids produced therein (tubing). Casing and tubing are made and transported in relatively short lengths and installed in the borehole one length at a time, each length being connected to the next. As the search for oil and gas has driven companies to drill deeper and more difficult wells, the demands on the casing and tubing have grown proportionately greater in terms of both tensile and pressure forces. The developing technology of deviated and horizontal wells have exacerbated this trend, adding to the casing and tubing requirements a further consideration of increasing torsional loads.
Two general classes of connectors exist within this field. The most common is the threaded and coupled connector, wherein two pin, or male threads, which are machined on the ends of two long joints of pipe, are joined by two box, or female threads, machined on a relatively short member, a coupling, with a larger outside diameter than the pipe, and approximately the same inside diameter. The other class is the integral connector, wherein the pin member is threaded onto one end of a full-length joint of pipe and the box member is threaded into the second full-length joint. The two joints can then be directly joined without the need for an intermediate coupling member. The ends of the pipe body may be processed further to facilitate the threading of the connection.
A thread profile is generally defined by a thread root, a thread crest, a stab flank, and a load flank as generally shown in FIG. 1. In a conventional thread, the “included angle”, the angle between the load and stab flanks is positive, meaning that the width of the thread crest is less than the width of the thread groove with which it is initially engaged. Hence, the pin tooth is easily positioned into the box groove as the threads are assembled by rotating one member into the other. In the final assembly position, either or both of the crests and roots may be engaged, and clearance may exist between the load flanks or the stab flanks. This allows the thread to be easily assembled. As reflected in the exemplary thread position shown in FIGS. 2A (stab position), 2B (engaged position) and 2C (fully made-up position), this clearance avoids the case of the load and stab flanks developing positive interference with its mating surface, which would cause the thread to “lock” and not fully engage.
A number of advancements over the years have given rise to “premium” connections. One can generally characterize these connections, compared to the connections specified by API (American Petroleum Institute) and other like organizations, in that they feature: 1), more sophisticated thread profiles; 2), one or more metal-to-metal sealing surfaces; and 3), one or more torque shoulders. The torque shoulder(s) are a mechanism used to geometrically position the metal seal(s) and to react against the threads to resist externally applied torque, while maintaining relatively low circumferential stress within the threaded section(s) of the connection. The torque resistance is a function of the torque shoulder area.
Another type of thread system that has been used in this field is known as a “wedge” thread, which is formed by a system of dovetail threads of varying width or varying pitch. This type of thread arrangement allows threads to easily be engaged and assembled, and yet to develop positive interference between opposing flanks of the thread in the fully assembled position. The wedge thread generally has a greater torque resistance than other premium threaded connections. The “wedge thread” has certain disadvantages, the principal one being that it is far more difficult to manufacture and measure than a thread with only a single pitch. Manufacturing a wedge thread on a taper further increases the difficulty of both the threading process and the measurement process.
What is needed by the drillers and producers of deep, high-pressure, hot, and/or deviated oil and gas wells is a threaded connection that has high-torque characteristics with relative ease of machining and production cost.